1. Field of the Invention
The present invention relates to a display device and a demultiplexer, and more particularly to an organic electroluminescent display and a demultiplexer, in which a demultiplexer includes a demultiplexing circuit including a sample/hold circuit and a pre-charge switching circuit.
2. Discussion of Related Art
An organic electroluminescent display is based on a phenomenon that an exciton emits light of a specific wavelength in an organic thin film, wherein the exciton is formed by recombination of an electron and a hole injected from a cathode and an anode, respectively. The organic electroluminescent display includes a self-emitting device, unlike a liquid crystal display (LCD), so that a separate light source is not needed. In the organic electroluminescent display, the brightness of an organic electroluminescent device varies according to the quantity of current flowing through an organic light-emitting device or organic light-emitting diode (OLED).
The organic electroluminescent display can be classified as a passive matrix type or an active matrix type according to its driving method. In the case of the passive matrix type, the anode and the cathode are perpendicularly disposed and form a line to be selectively driven. The passive matrix type organic electroluminescent display can be easily realized due to a relatively simple structure, but is not suitable for realizing a large-sized screen because it consumes much more power and the time allotted to drive each light-emitting device is shortened. On the other hand, in the case of the active matrix type, an active device is used to control the quantity of current flowing through the light-emitting device. As the active device, a thin film transistor (hereinafter, referred to as “TFT”) is widely used. The active matrix type organic electroluminescent display has a relatively complicated structure, but it consumes relatively little power and the time allotted to drive each organic electroluminescent device is relatively longer.
Hereinbelow, a conventional organic electroluminescent display will be described with reference to FIGS. 1 and 2.
FIG. 1 is a view showing a conventional organic electroluminescent display having an active matrix of n×m pixels.
Referring to FIG. 1, a conventional organic electroluminescent display includes a panel 11, a scan driver 12, and a data driver 13. The panel 11 includes n×m pixels 14, n scan lines SCAN[1], SCAN[2], . . . , SCAN[n] formed horizontally, and m data lines DATA[1], DATA[2], . . . , DATA[m] formed vertically, where n and m are natural numbers. Here, the scan driver 12 transmits scan signals to the pixels 14 through the scan lines SCAN[1] to SCAN[n], and the data driver 23 applies data voltages to the pixels 14 through the data lines DATA[1] to DATA[m].
FIG. 2 is a circuit diagram of a pixel employed in the organic electroluminescent display of FIG. 1. In FIG. 2, DATA represents one of the data lines of FIG. 1, and SCAN represents one of the scan lines of FIG. 1.
Referring to FIG. 2, a pixel of a conventional organic electroluminescent display includes an organic light emitting device OLED, a driving transistor MD, a capacitor C, and a switching transistor MS. The driving transistor MD is connected to the organic light emitting device OLED, and supplies a current to the organic light emitting device to emit light. Further, the switching transistor MS applies a data voltage to control the quantity of current supplied by the driving transistor MD. Further, the capacitor C is connected between a source and a gate of the driving transistor MD, and maintains a voltage corresponding to the data voltage applied by the switching transistor MS for a predetermined period.
With this configuration, when a scan signal is applied to a gate of the switching transistor MS and thus the switching transistor MS is turned on, the data voltage is applied to the gate of the driving transistor MD through the data line DATA. Accordingly, as the data voltage is applied to the gate of the driving transistor MD, the driving transistor MD supplies a current to the organic light emitting device OLED, thereby allowing the organic light emitting device OLED to emit light.
At this time, the current flowing through the organic light emitting device OLED is based on the following Equation 1.IOLED=ID=(β/2)(VGS−VTH)2=(β/2)(VDD−VDATA−|VTH|)2,  [Equation 1]where IOLED is a current flowing through the organic light emitting device, ID is a current flowing from the source to a drain of the driving transistor MD, VGS is a voltage applied between the gate and the source of the driving transistor MD, VTH is a threshold voltage of the driving transistor MD, VDD is a power voltage, VDATA is a data voltage, and β is a gain factor.
Referring back to FIG. 1, in the conventional organic electroluminescent display, the data driver 13 is directly connected to the data lines of the pixels. Therefore, when the number of data lines is increased, the data driver 13 becomes more complex in proportion to the number of data lines. On the other hand, even though the data driver 13 is realized as a chip separately from the panel 11, when the number of data lines is increased, the number of pins for the data driver 13 and the number of interconnection lines connecting the data driver 13 and the panel 11 should be increased in proportion to the number of data lines, thereby increasing production costs and circuit mounting space needed.
The current driving method can be classified as a voltage programming type or a current programming type. In the case of a current programming type pixel circuit, there is an advantage that display characteristics such as brightness are substantially uniform as long as the power source substantially uniformly supplies current to a pixel circuit even though the driving transistors for the respective pixels have different voltage-current property from each other.
However, in the current programming type pixel circuit in which the current is used as an input data signal for the pixel, voltage charged in a parasitic capacitor of the data line DATA by a data current of a preceding pixel line has an effect on the data programming time. Therefore, particularly, in the case of low gradation, data programming speed is lowered.